WASHINGTON — The Food and Drug Administration announced Tuesday that it would scrap a decades-old lifetime prohibition on blood donation by gay and bisexual men, a major stride toward ending what many had seen as a national policy of discrimination.

However, the agency will continue to ban men who have been sexually active in the last year, saying that the barrier is necessary to keep the blood supply safe, a move that frustrated rights groups that were pushing for the ban to be removed entirely.

The FDA enacted the ban in 1983, early in the AIDS epidemic. At the time, little was known about the human immunodeficiency virus, which causes the disease, and there was no quick test to determine whether somebody had it. But science — and the understanding of HIV in particular — has advanced in the intervening decades.

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The new 12-month waiting period imposes on gay men the same restriction as that imposed on heterosexuals who engage in high-risk behavior, including sex with prostitutes or with people who inject drugs.

Although the most modern tests can detect the virus in blood in as little as nine days after infection, the restrictions on heterosexuals were written before the tests became that refined, when it might take several weeks before the virus or antibodies to it could be detected. Also, blood banks know that donors do not always tell the truth about their sexual activity on questionnaires. A yearlong prohibition on dangerous sexual activity gives them a larger margin of error.

In written remarks, the agency said it was keeping the 12-month ban because “compelling scientific evidence is not available at this time to support a change to a deferral period less than one year while still ensuring the safety of the blood supply.”

The shift puts the United States on par with many European countries, including Britain, which adjusted its lifetime ban in favor of a 12-month restriction in 2011.

The FDA rules on blood donation generally include very wide margins of error. For example, it bars anyone who has traveled in areas where malaria is common from giving blood for a year, even though malaria symptoms are almost unmistakable — chills and fever — and virtually always appear within 40 days.

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Most men’s health advocates called the FDA’s move long overdue, and said the overall ban was not based on the latest science, and perpetuates stigma about gay men as a risk to the health of the nation. Legal experts said the change brought an important national health policy in line with other legal and political rights for gay Americans, like permitting gay people to marry and to serve openly in the military.

“This is a major victory for gay civil rights,” said I. Glenn Cohen, a law professor at Harvard who specializes in bioethics and health. “We’re leaving behind the old view that every gay man is a potential infection source.” He said, however, that the policy was “still not rational enough.”

Some advocacy groups attacked the change as too incremental. Leaving in place a 12-month ban essentially blocks any gay or bisexual man who is sexually active from donating, erasing about half the population of potential donors and perpetuating what rights groups say is tougher treatment for gay and bisexual men.

GMHC, an advocacy group formerly known as Gay Men’s Health Crisis, called the new policy “offensive and harmful.” AIDS United, a Washington-based group, said it was a “step forward,” but it “continues to perpetuate discrimination against gay and bisexual men.”

Other groups applauded the shift, pointing out that it had been pushed for years by medical experts, blood banks and gay men’s health organizations who argued that the policy had long outlived its usefulness for safety in the blood supply.

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In a statement, the US agency said that it had “carefully examined and considered the scientific evidence” and that it intended to issue a draft guidance detailing the change next year. An FDA official told reporters there was not enough science to support lifting the 12-month ban.